1. Field of the Invention
The present invention relates to a meter electronics and methods for verification diagnostics for a flow meter.
2. Statement of the Problem
Vibrating conduit sensors, such as Coriolis mass flow meters or vibrating tube densitometers, typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the material in the conduit, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness, and damping characteristics of the containing conduit and the material contained therein.
A conduit of a vibratory flow meter can include one or more flow tubes. A flow tube is forced to vibrate at a resonant frequency, where the resonant frequency of the tube is proportional to the density of the fluid in the flow tube. Sensors located on the inlet and outlet sections of the tube measure the relative vibration between the ends of the tube. During flow, the vibrating tube and the flowing mass couple together due to Coriolis forces, causing a phase shift in the vibration between the ends of the tube. The phase shift is directly proportional to the mass flow.
A typical Coriolis mass flow meter includes one or more conduits that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries and the like, in the system. Each conduit may be viewed as having a set of natural vibration modes including, for example, simple bending, torsional, radial, and coupled modes. In a typical Coriolis mass flow measurement application, a conduit is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit. Excitation is typically provided by an actuator, e.g., an electromechanical device, such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. Mass flow rate may be determined by measuring time delay or phase differences between motions at the transducer locations. Two such transducers (or pickoff sensors) are typically employed in order to measure a vibrational response of the flow conduit or conduits, and are typically located at positions upstream and downstream of the actuator. The two pickoff sensors are connected to electronic instrumentation by cabling. The instrumentation receives signals from the two pickoff sensors and processes the signals in order to derive a mass flow rate measurement.
The phase difference between the two sensor signals is related to the mass flow rate of the material flowing through the flow tube or flow tubes. The mass flow rate of the material is proportional to the time delay between the two sensor signals, and the mass flow rate can therefore be determined by multiplying the time delay by a Flow Calibration Factor (FCF), where the time delay comprises a phase difference divided by frequency. The FCF reflects the material properties and cross-sectional properties of the flow tube. In the prior art, the FCF is determined by a calibration process prior to installation of the flow meter into a pipeline or other conduit. In the calibration process, a fluid is passed through the flow tube at a given flow rate and the proportion between the phase difference and the flow rate is calculated.
One advantage of a Coriolis flow meter is that the accuracy of the measured mass flow rate is not affected by wear of moving components in the flow meter. The flow rate is determined by multiplying the phase difference between two points of the flow tube and the flow calibration factor. The only input is the sinusoidal signals from the sensors, indicating the oscillation of two points on the flow tube. The phase difference is calculated from these sinusoidal signals. There are no moving components in the vibrating flow tube. Therefore, the measurement of the phase difference and the flow calibration factor are not affected by wear of moving components in the flow meter.
The FCF can be related to a stiffness characteristic of the meter assembly. If the stiffness characteristic of the meter assembly changes, then the FCF will also change. Changes therefore will affect the accuracy of the flow measurements generated by the flow meter. Changes in the material and cross-sectional properties of a flow tube can be caused by erosion or corrosion, for example. Consequently, it is highly desirable to be able to detect and/or quantify any changes to the stiffness of the meter assembly in order to maintain a high level of accuracy in the flow meter.